1. Technical Field
The present invention concerns a method and a device for the programmable time profile shaping of quasi-monochromatic optical pulses.
2. State of the Prior Art
In certain applications that use pulsed laser sources, it is necessary to shape the time profile of the amplitude of the optical pulses. Examples that may be cited include the field of power lasers, or the telecommunications field, where it is necessary to shape the profile of pulses in the time domain before transmitting information.
Procedures for shaping the time profile of optical pulses aim to satisfy several criteria simultaneously:
To obtain time profile shaping with good resolution, with preferably 100 to 1,000 points over the whole length of the pulse, with each point being able to reach the femtosecond domain.
To be programmable, in other words to allow a change in the shape of the pulse both when desired and rapidly, for example in an automatic manner.
To be compatible with the production of pulses that have spectra that are as narrow as the Fourier limit of the created pulses.
To produce pulses that can be wavelength tuned.
To not cause too much energy loss.
Basically, two general approaches are used to shape the time profile of laser pulses. The first uses essentially optical methods. The second consists in converting the electronic signals into optical signals, with the corresponding procedures using electro-optical or acousto-optical systems.
Procedure For Time Shaping by Spectral Modulation
An entirely optical modulation procedure of this type is described in the document reference (1) at the end of the description. In this procedure, as shown in FIG. 1, a beam of light 10 from a source point 01, which is collimated, is diffracted by an optical grating 11 so that it can then be focused at point 02. The pupil 12 is located in plane P of the grating 11. When a short pulse is diffracted through the pupil in a given direction, it is possible to construct a time profile h (t) by applying a primitive function of h (t) as a transmitting function using the mask 13, in the plane of the pupil 12 following the direction x. A spatial filter 14 is placed at the point 02 in order to obtain a spectrally homogeneous beam. Such a device is similar to a spectrometer. The time resolution is identical to the length of the injected short pulse. The length of the output pulse is equal to the relative maximum time delay of each of the rays of the beam that covers the pupil 12 in the plane P.
This procedure enables good shaping performance of the time profile to be obtained, particularly as regards the number of desired points. It is programmable: a variable spatial filter only has to be placed in the plane of the pupil. It produces quasi-monochromatic pulses. In addition, it allows a tuneable wavelength to be produced. On the other hand, it has very low energy yield, around the inverse of the number of resolved points.
Procedure For Time Shaping by Fourier Transform
The fields in the frequency range and in the time range are linked by the Fourier transform E(v)=TF[E (t)].
If one wishes the time profile A (t) of the pulse to lie within the Fourier limits, the field must posses a linear phase with the spectral variable. The pulse is then quasi-monochromatic. By acting on the amplitude and the incident pulse phase, it is possible to modify its time profile. The spectral mask M (v) in amplitude and in phase must then satisfy the equation: XXX, where X is the incident spectral field of the device. The procedure therefore consists in simple spectral selection (in amplitude and in phase), providing that the incident spectral field has all of the spectral components of the field to produce (in other words, if (M(v)) less than 1).
In the device shown in FIG. 2, initially one has a short pulse with a wide spectrum. The spectral amplitude is modified using a spectral selection device 20, made out of spectral amplitude and phase filters. A time profile is then obtained, which is the Fourier transform of the spectral amplitude which has been shaped in the spectral plane.
It can be seen that the spectral selection is achieved in a similar manner for the majority of optical systems. One only has to place a spatial filter in a specific plane of the spectral selection device. It is then possible to achieve programmable shaping by using a variable transmission spatial system.
This Fourier transform time shaping procedure gives goods results as regards the number of resolved points since the desired resolutions may be obtained. It is also programmable, since it uses spectral selection. This procedure also enables a narrow spectrum to be generated, and the wavelength is, in addition, tuneable: one only has to displace the mask in the spectral plane in order to change the wavelength. Although the pulse length is variable, it is dependent on the spectral width of the pulse produced (via the intermediary of the Fourier transform). It is not therefore possible to obtain long pulses. The main disadvantage of this type of procedure is, in fact, the energy yield. As a matter of fact, time shaping with good resolution in the spectral plane requires considerable losses in energy if a narrow spectrum is desired, since a large part of the spectrum must be cut off. This procedure is, as a result, only used very infrequently.
Procedure For Time Shaping Pulses With Frequency Drift
This type of procedure is described in document (2). FIG. 3 shows a schematic diagram of this procedure. The pulses are schematically represented in order to show their length and their spectral width. Time shaping by completely optical means may be achieved by using short pulses. Such pulses, which have wide spectra, are frequency drifted after going through a dispersion device 22, as in that described in document (3). The wavelengths are then time dispersed, which involves spreading out the wavelengths that make up the wide spectrum over time. By selecting the wavelengths using a spectral selection device 23, time shaping is carried out.
Very good resolution may be achieved using this procedure, with the number of resolved points greater than 100. This procedure allows programmable time shaping: it consists, in effect, in transferring the shaping into the spectral range and a large number of programmable spectral selection systems are available, such as that described in document (4). The energy loss caused by this type of procedure is minimal, since only the essential components of the pulse are lost in time shaping. However, the shaped pulse has a wide spectrum.
It can be seen that procedures that use a spectral selection device 23 to carry out the shaping of the time profile of a pulse, whose means of operation is schematically shown in FIG. 4, generally all operate on the basis of a spectrometer. The resolving power is, by definition, the ratio between the central wavelength of the device xcex and the spectral resolution xcex4xcex:R=xcex/xcex4xcex. The number, P, of time resolved points is defined by T1/T2 and is given by the equation:   P  =                    Δ        ⁢                  xe2x80x83                ⁢        λ            λ        ⁢                  λ                  δ          ⁢                      xe2x80x83                    ⁢          λ                    .      
The quantity xcex94xcex/xcex is a characteristic of the incident laser pulse in the system. The order of magnitude of the xcex94xcex/xcex quantities is from 10xe2x88x922 to 10xe2x88x921 depending on the wavelengths used. The resolving power is conventionally around 104. This type of device therefore allows a number of resolved points of between 100 and 1,000 to be obtained.
These procedures achieve time profile shaping by converting the electronic signals into optical signals and obtaining an interference between the non-modified part of an optical impulsion and the modified part of this optical pulse using an electrical signal.
FIG. 5 shows a schematic diagram of this type of procedure. A field E0 with intensity I0 is separated into two parts. The two fields Ea and Eb follow different paths, and one of the fields is de-phased in a de-phaser 27. Their sums are then combined so that they interfere with each other. The phase shift varies with the time. One thus obtains the equation: ET=Ea+Ebeixcfx86(t). In illuminance, one then obtains the equation:       I    T    =                              I          0                2            ⁡              [                  1          +                      cos            ⁢                          xe2x80x83                        ⁢                          ϕ              ⁡                              (                t                )                                                    ]              .  
This phase shift may be achieved with an electro-optical element, for example by using the Pockels effect, as described in document (5). Several means are available to shape the applied electrical signal, but they all have the same limitations. In effect, the range of frequencies that can be accessed by an electrical system is limited to several gigahertz, which is much less than the optical frequencies (around 100 THz). This type of procedure thus always has limited resolution: several tens of points at the most over the whole length of the pulse in the case of, for example, nanosecond pulses. It is, moreover, extremely difficult to obtain the desired electrical signal forms. The programmable methods that are available are based on the combination of several electrical signals. For example, the combination of several Gaussians or Heaviside functions of different intensities are used in order to create the desired signal, but the downside is that there are always residual amplitude modulations in the optical signal. The only alternative that can be used to improve these modulations and the resolution is to increase the number of electro-optical components, which adds to the number of disadvantages in terms of cost, bulk and ease of use. It is also possible to limit the modulations by filtering the electrical signal. However, this filtering reduces the resolution of the procedure. A compromise then has to be found between the desired resolution and the modulations.
These electro-optical procedures are, nevertheless, the most frequently used. They operate with pulses that have narrow spectra and any length, providing they remain above one nanosecond. They are also be programmable, by acting on the electrical signal. In addition, they do not lead to too high energy losses. However, these procedures do not make the wavelength tuneable and have limited resolution.
None of the procedures of the prior art therefore allow all the desired criteria to be met. The optical procedures generally allow very good performance to be obtained in terms of resolution, but they can only be used with pulses with wide spectra and are costly in terms of energy. Electro-optical procedures allow monochromatic pulses to be created, but do not provide the performance of completely optical shaping.
An article of the prior art entitled xe2x80x9cEfficient frequency tripling of 1.06 xcexcm/300 fs chirped pulsesxe2x80x9d by F. Raoult et al, published in CLEO"" 98, page 523, makes it possible to frequency triple wide band width pulses by using a compressed pulse amplification (CPA) configuration. As shown in the embodiment in FIG. 2, two pulses that are compressed differently in a CPA system or coming out of a wide, beam splitting output of a CPA amplifier, are sent on two stages of pairs of different gratings.
An article of the prior art entitled xe2x80x9cEfficient generation of narrow band width picosecond pulses by frequency doubling of femtosecond chirped pulsesxe2x80x9d by F. Raoult et al, published in Optics Letters, volume 23 (1998), pages 1117 to 1119, describes the generation of narrow band width pulses in picoseconds by frequency mixing of two opposite compressed pulses with wide band width in a type I doubling crystal.
The object of the present invention is therefore to resolve the problems associated with the procedures of the prior art, while making it possible to meet all of the various criteria described above.
The present invention concerns an optical process for the programmable time profile shaping of quasi-monochromatic optical pulses, comprising the following steps:
(1) the spectral components of a wide spectrum pulse are spread out over time and a stretched pulse is obtained while at the same time conserving spectral width;
(2) the pulse is spectrally shaped and, as a consequence, time shaped; and
(3) the wide spectrum is converted into a narrow spectrum while conserving time shaping.
In one embodiment:
two input pulses are provided;
these two pulses are stretched;
one of the two pulses is shaped; and
the frequencies of these two pulses are mixed by combining the sums or the differences of the frequencies of these two pulses.
In a first example of the embodiment:
two pulses, whose frequency drifts have opposite signs, are provided, with these two pulses being such that: xcfx891(t)=xcfx890+2xcfx86xe2x80x2(t) and xcfx892(t)=xcfx890xe2x88x92xcfx86xe2x80x2(t);
one of the two pulses is time shaped; and
the sums of the frequencies of the two pulses are combined in order to obtain a time shaped, monochromatic pulse, such that: xcfx893(t)=xcfx892(t)+xcfx891(t)=2xcfx890.
In a second example of the embodiment:
two input pulses, whose frequency drifts have the same signs, are provided, with these two pulses being such that: xcfx891(t)=xcfx890+2xcfx86xe2x80x2(t) and xcfx892(t)=xcfx890+xcfx86xe2x80x2(t);
one of the two pulses is time shaped;
the second pulse is converted into is second harmonic; and
the difference of the frequencies of the two pulses are combined in order to obtain a time shaped, monochromatic pulse, such that: xcfx893(t)=xcfx892(t)xe2x88x92xcfx891(t)=xcfx890.
The process according to the invention fully meets all of the desired requirements. In fact, it benefits from excellent shaping performance of the optical time profile. It produces a narrow spectrum pulse and, in addition, benefits from a very advantageous aspect: it offer, in fact, the possibility of obtaining a variable wavelength over several nanometers. Moreover, its energy yield is good, sine it mainly depends on frequency conversion, and the pulse may be adapted (in terms of intensity) to the conversion procedure in such a way as to obtain excellent yield.
The present invention also concerns an optical device for the programmable time shaping of quasi-monochromatic optical pulses, comprising:
a dispersion device; and
a spectral selection device;
whereby it comprises, in addition, a device for converting wide spectra into narrow spectra.
In one embodiment, this device comprises:
a first path that comprises a first dispersion device followed by a special selection device;
a second path that comprises a second dispersion device;
a device for mixing the output signals from these two paths;
and, if appropriate, a frequency modulation device placed in one of the two paths.